Defense attorneys tried to convince the jury that Bulger was not an FBI informant, a notion prosecutors called "ludicrous" in light of his FBI informant card and a 700-page file loaded with "tips" on rival gang members.
In closing arguments, defense attorney Hank Brennan told jurors why he thinks the government has made Bulger's informant status the crux of its case.
"Think about why it's so important. If it's not an issue, why do they keep bringing it up?" Brennan asked.
Brennan explained that other than to "embarrass" his client, the government using its claim that Bulger was an informant to cover up years of corruption.
"If he's not an informant, think of the liability," he said, recounting instance after instance in which government officials as high as strike-force attorneys in the Justice Department protected Bulger throughout his criminal career.
A number of retired FBI agents and supervisors took the stand during the trial, many testifying that they believed Bulger should have been shut down as an informant because he wasn't providing any useful information.
The agents said they never pressed the issue because apparently FBI headquarters felt Bulger was useful in taking down the New England Mafia.
It took prosecutors 90 minutes to detail the 19 murders Bulger is accused of, showing photos of each of the victims and the crime scenes.
Bulger is not charged with delivering the fatal blow in all of the murders, but is charged with participating as part of a racketeering conspiracy. Wyshak called Bulger "the leader of a very wide-ranging, broad organization," who is culpable for his co-conspirator's crimes.
But Wyshak made clear Bulger was the alleged triggerman in some cases, recalling testimony from Bulger's partner, Steve "The Rifleman" Flemmi, about the murder of John McIntyre, whose remains were exhumed from a shallow makeshift grave in 2000.
McIntyre had begun cooperating with the government on the shipment of weapons to the Irish Republican Army and 36 tons of marijuana imported into Boston Harbor.
"It's Arthur Barrett all over again, held at gunpoint, chained to the kitchen chair, interrogated," Wyshak said.
Flemmi said he was holding McIntyre's body while Bulger was trying to strangle him with a rope, but the rope was too thick.
After that didn't work, McIntyre practically begged for a bullet after Bulger asked him if he would "like one in the head," according to Flemmi.
Flemmi testified as one of the government's star witnesses, and he said that he saw Bulger strangle the two women. The defense team, however, presented evidence that Flemmi had the greater motive to kill the women -- his girlfriend and his stepdaughter.
The girlfriend, Debra Davis, was about to leave him for another man. The defense recalled Martorano, who testified that Flemmi admitted he "accidentally strangled" the 26-year-old woman.
Flemmi acknowledged he lured Davis to a home but says Bulger strangled her because she was talking too much and had become a liability.
Wyshak recalled testimony that Bulger always needed to take a nap after strangling or shooting his victims to death.
In talking about the murder of Paul McGonagle, Wyshak said Bulger's former cohorts testified that whenever they passed the Neponset River, where McGonagle's remains were exhumed in 2000, Bulger said, "'Drink up, Paulie.' That's the level of humanity that this defendant is operating at. ... And every time he goes by there it's 'Drink up, Paulie.'"
'Bulger: I didn't get a fair trial'
Bulger was a fugitive for more than 16 years, after a crooked FBI agent told him in December 1994 he was about to be indicted on federal racketeering charges. The FBI tracked him down and arrested him 2011 in Santa Monica, California, where he had been living with his girlfriend under an alias.
Bulger never took the stand, despite repeated hints from his lawyers throughout trial he would testify. In fact, Bulger seemed to want to testify, and when questioned about it by the judge, Bulger called his decision a "choice made involuntarily."
He claimed he had been given immunity for his crimes by the former head of New England's Organized-Crime Strike Force, Jeremiah O'Sullivan, now deceased.
Bulger, who lost his temper several times during the trial, appeared angry, shaking his finger at the judge and claiming he was "choked off from making an adequate defense."